FINALLY!!! It's the feature that a lot people have been asking for.
Amazon Echo's new firmware has a sleep timer.
So if you like to listen to music to help you fall asleep, you can set a sleep timer to turn off the music after a certain number of minutes.
Just say something like:
Alexa, play JWLV's playlist.
Alexa, set a sleep timer for 45 minutes.
That's it!